Networks of conduits are found in many industrial applications. While it is common to provide a valve to control flow through a conduits it is sometimes required to completely isolate pipes, tanks, or like equipment for cleaning, maintenance, modification, and so forth. For safety reasons a blind plate is provided, commonly at a flanged connection and secured in position to completely block off the conduit and prevent the passage of fluid, either liquid or gas as the case may be, therethrough. In a simple blind plate device, a flanged connection is opened sufficiently, by removing the required number of bolts and prying the flanges apart if necessary, to allow a blind plate to be placed between the two flanges. The bolts are then re-inserted and tightened. A typical blind plate device has a first portion that defines a flow aperture substantially equal in diameter to the conduit, and a second portion that is solid and blocks flow through the conduit. Thus it is readily apparent on viewing the flanged connection with the blind plate installed whether the device is oriented such that the conduit is open or closed.
Typically such flanged connections include a seal between the flanges. When the blind plate is used, a seal is required on each side of the blind plate to seal the blind plate to each flange. When changing the location of the blind plate it is necessary to ensure that the seals are properly oriented and fitted so that they seal properly when the flanges are re-tightened. This requirement adds time to the procedure, and improper fitting may require that the procedure be re-done.
Similar types of blind plates are described in United States Published Patent Application 2003/0056843 of Carey and in U.S. Pat. No. 6,085,793 to Mayhew. The blind plate described in U.S. Pat. No. 5,582,211 to Monson is also adapted for purging a conduit.
Blind plates are also known where a valve plate body is provided in a conduit and the blind plate has a solid closed portion and an open portion defining an aperture typically the same diameter as the conduit. The blind plate can slide in the valve plate body from an open position, where the open portion is aligned with the conduit, to a closed position where the closed portion blocks the conduit. Such a device is illustrated for example in U.S. Pat. No. 3,695,299 to Rodgers and in U.S. Pat. No. 7,017,886 to Ngene-Igwe.
It is also sometimes desired, such as for gas flow measurement, to temporarily restrict the flow of fluid through a conduit by providing an orifice in the conduit that is smaller than the inside of the conduit. U.S. Pat. No. 5,160,119 to Lemire et al. discloses a slide plate apparatus that provides an orifice that can be made smaller or larger by moving a pair of slide plates relative to each other.